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From: Will Deacon <will.deacon@arm.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: linux-kernel@vger.kernel.org, Jason Wang <jasowang@redhat.com>,
	Alan Stern <stern@rowland.harvard.edu>,
	Andrea Parri <andrea.parri@amarulasolutions.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Boqun Feng <boqun.feng@gmail.com>,
	Nicholas Piggin <npiggin@gmail.com>,
	David Howells <dhowells@redhat.com>,
	Jade Alglave <j.alglave@ucl.ac.uk>,
	Luc Maranget <luc.maranget@inria.fr>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	Akira Yokosawa <akiyks@gmail.com>,
	Daniel Lustig <dlustig@nvidia.com>,
	linux-arch@vger.kernel.org, netdev@vger.kernel.org,
	virtualization@lists.linux-foundation.org,
	Luc Van Oostenryck <luc.vanoostenryck@gmail.com>,
	linux-sparse@vger.kernel.org
Subject: Re: [PATCH RFC 2/4] include/linux/compiler.h: allow memory operands
Date: Mon, 7 Jan 2019 17:54:27 +0000	[thread overview]
Message-ID: <20190107175427.GB22911@fuggles.cambridge.arm.com> (raw)
In-Reply-To: <20190102205715.14054-3-mst@redhat.com>

On Wed, Jan 02, 2019 at 03:57:54PM -0500, Michael S. Tsirkin wrote:
> We don't really care whether the variable is in-register
> or in-memory. Relax the constraint accordingly.
> 
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>  include/linux/compiler.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index 1ad367b4cd8d..6601d39e8c48 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -154,7 +154,7 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
>  #ifndef OPTIMIZER_HIDE_VAR
>  /* Make the optimizer believe the variable can be manipulated arbitrarily. */
>  #define OPTIMIZER_HIDE_VAR(var)						\
> -	__asm__ ("" : "=r" (var) : "0" (var))
> +	__asm__ ("" : "=rm" (var) : "0" (var))
>  #endif

I think this can break for architectures with write-back addressing modes
such as arm, where the "m" constraint is assumed to be evaluated precisely
once in the asm block.

Will

WARNING: multiple messages have this Message-ID (diff)
From: Will Deacon <will.deacon@arm.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Andrea Parri <andrea.parri@amarulasolutions.com>,
	linux-arch@vger.kernel.org,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Daniel Lustig <dlustig@nvidia.com>,
	Akira Yokosawa <akiyks@gmail.com>,
	linux-kernel@vger.kernel.org, Nicholas Piggin <npiggin@gmail.com>,
	virtualization@lists.linux-foundation.org,
	David Howells <dhowells@redhat.com>,
	linux-sparse@vger.kernel.org,
	Alan Stern <stern@rowland.harvard.edu>,
	netdev@vger.kernel.org, Luc Maranget <luc.maranget@inria.fr>,
	Jade Alglave <j.alglave@ucl.ac.uk>,
	Boqun Feng <boqun.feng@gmail.com>,
	Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Subject: Re: [PATCH RFC 2/4] include/linux/compiler.h: allow memory operands
Date: Mon, 7 Jan 2019 17:54:27 +0000	[thread overview]
Message-ID: <20190107175427.GB22911@fuggles.cambridge.arm.com> (raw)
In-Reply-To: <20190102205715.14054-3-mst@redhat.com>

On Wed, Jan 02, 2019 at 03:57:54PM -0500, Michael S. Tsirkin wrote:
> We don't really care whether the variable is in-register
> or in-memory. Relax the constraint accordingly.
> 
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>  include/linux/compiler.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index 1ad367b4cd8d..6601d39e8c48 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -154,7 +154,7 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
>  #ifndef OPTIMIZER_HIDE_VAR
>  /* Make the optimizer believe the variable can be manipulated arbitrarily. */
>  #define OPTIMIZER_HIDE_VAR(var)						\
> -	__asm__ ("" : "=r" (var) : "0" (var))
> +	__asm__ ("" : "=rm" (var) : "0" (var))
>  #endif

I think this can break for architectures with write-back addressing modes
such as arm, where the "m" constraint is assumed to be evaluated precisely
once in the asm block.

Will

  reply	other threads:[~2019-01-07 17:54 UTC|newest]

Thread overview: 94+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-01-02 20:57 [PATCH RFC 0/4] barriers using data dependency Michael S. Tsirkin
2019-01-02 20:57 ` Michael S. Tsirkin
2019-01-02 20:57 ` [PATCH RFC 1/4] include/linux/compiler*.h: fix OPTIMIZER_HIDE_VAR Michael S. Tsirkin
2019-01-02 20:57   ` Michael S. Tsirkin
2019-01-02 20:57   ` Michael S. Tsirkin
2019-01-08 17:44   ` Nick Desaulniers
2019-01-08 17:44     ` Nick Desaulniers
2019-01-08 17:44     ` Nick Desaulniers
2019-01-08 18:50     ` Michael S. Tsirkin
2019-01-08 18:50       ` Michael S. Tsirkin
2019-01-08 18:50       ` Michael S. Tsirkin
2019-01-08 18:50     ` Michael S. Tsirkin
2019-01-09 10:35     ` Miguel Ojeda
2019-01-09 10:35       ` Miguel Ojeda
2019-01-09 10:35       ` Miguel Ojeda
2019-01-09 14:50       ` Michael S. Tsirkin
2019-01-09 14:50         ` Michael S. Tsirkin
2019-01-09 14:50         ` Michael S. Tsirkin
2019-01-19 18:35         ` Miguel Ojeda
2019-01-19 18:35           ` Miguel Ojeda
2019-01-20 14:43           ` Michael S. Tsirkin
2019-01-20 14:43           ` Michael S. Tsirkin
2019-01-20 14:43             ` Michael S. Tsirkin
2019-01-20 15:36             ` Miguel Ojeda
2019-01-20 15:36               ` Miguel Ojeda
2019-01-09 14:50       ` Michael S. Tsirkin
2019-01-10  2:36       ` Michael S. Tsirkin
2019-01-10  2:36         ` Michael S. Tsirkin
2019-01-10  2:36         ` Michael S. Tsirkin
2019-01-10 13:41         ` Dan Carpenter
2019-01-10 13:41           ` Dan Carpenter
2019-01-10 13:41           ` Dan Carpenter
2019-01-10 14:08           ` Michael S. Tsirkin
2019-01-10 14:08             ` Michael S. Tsirkin
2019-01-10 14:08             ` Michael S. Tsirkin
2019-01-10 13:41         ` Dan Carpenter
2019-01-10  2:36       ` Michael S. Tsirkin
2019-01-02 20:57 ` Michael S. Tsirkin
2019-01-02 20:57 ` [PATCH RFC 2/4] include/linux/compiler.h: allow memory operands Michael S. Tsirkin
2019-01-02 20:57 ` Michael S. Tsirkin
2019-01-07 17:54   ` Will Deacon [this message]
2019-01-07 17:54     ` Will Deacon
2019-01-07 18:16     ` Michael S. Tsirkin
2019-01-07 18:16       ` Michael S. Tsirkin
2019-01-02 20:57 ` [PATCH RFC 3/4] barriers: convert a control to a data dependency Michael S. Tsirkin
2019-01-02 20:57   ` Michael S. Tsirkin
2019-01-02 20:57   ` Michael S. Tsirkin
2019-01-02 21:00   ` Matthew Wilcox
2019-01-02 21:00   ` Matthew Wilcox
2019-01-02 21:00     ` Matthew Wilcox
2019-01-02 21:00     ` Matthew Wilcox
2019-01-02 21:24     ` Michael S. Tsirkin
2019-01-02 21:24     ` Michael S. Tsirkin
2019-01-02 21:24       ` Michael S. Tsirkin
2019-01-02 21:24       ` Michael S. Tsirkin
2019-01-07  3:58   ` Jason Wang
2019-01-07  3:58     ` Jason Wang
2019-01-07  4:23     ` Michael S. Tsirkin
2019-01-07  4:23       ` Michael S. Tsirkin
2019-01-07  4:23       ` Michael S. Tsirkin
2019-01-07  4:23       ` Michael S. Tsirkin
2019-01-07  6:50       ` Jason Wang
2019-01-07  6:50         ` Jason Wang
2019-01-07  6:50         ` Jason Wang
2019-01-07  6:50         ` Jason Wang
2019-01-07  9:46       ` Peter Zijlstra
2019-01-07  9:46         ` Peter Zijlstra
2019-01-07 13:36         ` Michael S. Tsirkin
2019-01-07 13:36           ` Michael S. Tsirkin
2019-01-07 15:54           ` Peter Zijlstra
2019-01-07 15:54             ` Peter Zijlstra
2019-01-07 16:22             ` Michael S. Tsirkin
2019-01-07 16:22               ` Michael S. Tsirkin
2019-01-07 16:22               ` Michael S. Tsirkin
2019-01-07 16:22             ` Michael S. Tsirkin
2019-01-07 19:02           ` Paul E. McKenney
2019-01-07 19:02             ` Paul E. McKenney
2019-01-07 19:02             ` Paul E. McKenney
2019-01-07 19:13             ` Michael S. Tsirkin
2019-01-07 19:13             ` Michael S. Tsirkin
2019-01-07 19:13               ` Michael S. Tsirkin
2019-01-07 19:13               ` Michael S. Tsirkin
2019-01-07 19:25               ` Paul E. McKenney
2019-01-07 19:25                 ` Paul E. McKenney
2019-01-07 19:25                 ` Paul E. McKenney
2019-01-02 20:57 ` Michael S. Tsirkin
2019-01-02 20:58 ` [PATCH RFC 4/4] virtio: use dependent_ptr_mb Michael S. Tsirkin
2019-01-02 20:58 ` Michael S. Tsirkin
2019-01-02 21:36 ` [PATCH RFC 0/4] barriers using data dependency Alan Stern
2019-01-02 21:36   ` Alan Stern
2019-01-02 23:04   ` Michael S. Tsirkin
2019-01-02 23:04     ` Michael S. Tsirkin
2019-01-03 15:11     ` Alan Stern
2019-01-03 15:11       ` Alan Stern

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